Incorrect Index used randomly for the same Query

We recently encountered a production issue where the database randomly chooses different indexes for running a specific query. On certain days it uses the PK_A index whereas on certain days it chooses TUNE_B index. On days when the system uses the PK_A index, the query takes approximately one hour to finish. However, when the system choses the TUNE_A index, the query completes in under two seconds.

No. of Record in Table: 345,692,102

Index Name	Unique	Logging	Degree	Columns	Order
TUNE_V_REF_NO	N	NO	1	V_REF_NO	ASC
TUNE_BILL_TRANS	N	NO	1	V_POLICY_NO